1. Field of the Invention
The present invention relates to a digital information reproduction method of reproducing information from an optical disc medium having information recorded thereon by forming a record mark on the recording medium, the recording mark having a different physical property from the rest of the recording medium.
2. Description of the Related Art
Various types of optical disc media exist, such as CD-R/RWs, DVD-RAMs, DVD±R/RWs, Blu-ray Discs (hereafter simply mentioned as BDs) and HD DVDs. These optical disc media including those that have dual data layers are widely used for general purposes. An optical disc apparatus which is widely used and which supports these optical disc media is a so-called DVD super multi drive, which is capable of performing the recording/reproduction using CD-R/RWs, DVD-RAMs and DVD±R/RWs. More advanced drives supporting BDs or HD DVDs are expected to be popular in the foreseeable future.
Some of the already-proposed technologies related to the next-generation large-capacity optical discs include the super-resolution technique and the solid immersion lens (SIL). An example of the super-resolution technique is disclosed in the Japanese Patent Application Publication No. 2006-107588. According to the disclosed technique, the super-resolution reproduction is performed using phase-change recording films buried in pits. The phase-change recording film used here changes its optical properties when the film is melted. In addition, record marks are spatially-separated from one another to reduce the thermal interference between the record marks and to reduce fluctuation of the super-resolution region. According to this configuration, higher line density and higher track density can be obtained so that a significant increase in the recording capacity of the optical disc can be achieved. Moreover, larger transmittance in regions between data pits contributes to larger average transmittance of all over the data surface. Accordingly, higher light-use efficiency can be obtained. The discs thus configured are advantageous when the discs are made as a multi-layer disc. Hereinafter, the above-described type of disc will be called a three-dimensional pit selection type. One of the means available for the purpose of burying recording films in pits is a phase-change etching method (a method utilizing the difference in the etching speed between a crystalline material and an amorphous material) described in Japanese Patent Application Publication No. 2005-100526. Another available method for this purpose is a physical polishing processing method, such as the chemical mechanical polishing (CMP) method.
Technical Digest of Optical Data Storage 2007, TuC3 reports that the reproduction signal of super-resolution discs includes super-resolution signal components and normal resolution signal components. This document proposes a simulation method and a Viterbi decoding method utilizing the difference in bands between the two kinds of signal components.
Meanwhile, the reproduction of information from optical disc media including super-resolution discs inevitably involves a possible occurrence of errors in the digital data from one reason to another. One of the frequently-used methods of correcting these errors is the one using the error correcting codes, as represented by Reed-Solomon codes. In the Reed-Solomon correction, digital data is treated as symbol values by the byte, for example. Accordingly, as has been known widely, the Reed-Solomon codes are well-suited to applications where burst errors occur. Nevertheless, the error correction using the Reed-Solomon codes is performed for each symbol, so that the efficient error correction cannot be conducted by this method in a case where errors occur in the optical disc medium for a certain bit pattern with disproportionately high frequency. One of the error correction methods that are well-suited to application to such a case where errors are concentrated on a certain bit pattern is a method using parity check bits, and Technical Digest of Optical Data Storage 2003, TuB4 describes an example of such a method.
When it comes to the Viterbi decoding method, which the present invention focuses on, Japanese Journal of Applied Physics Vol. 45, No. 2B, 2006, pp. 1213-1218, describes a method of calculating edge shift amounts for all the edge patterns in accordance with run-length limited coding.